1. Field of the Invention
The present invention relates to an injection molding machine and, more particularly, to a switching control system for unit torque control value of a servo motor for an injection molding machine which uses a servo motor as a drive source for driving the injection mechanism.
2. Description of the Related Art
In a conventional injection molding machine, hydraulic pressure drives an injection mechanism and axially moves a screw to perform injection. An injection pressure and a back pressure could be adjusted by controlling the hydraulic pressure. On the other hand, injection molding machines in which a servo motor drives the injection mechanism to perform injection have been developed. In these injection molding machines driven by servo motors to perform injection, the injection pressure or the back pressure must be controlled by controlling the output torque of the servo motor. According to a conventional method of controlling the torque of a servo motor, the torque of the servo motor is limited. Although a back pressure during metering is as low as about 1/10 of a maximum injection pressure during injection, a torque limit command representing a large unit value is supplied to a D/A converter from a control unit. Therefore, it was impossible to set a small unit torque limit value during metering.